netdev - Re: [Patch net v3] tipc: call start and done ops directly in __tipc_nl_compat

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Date:   Wed, 5 Sep 2018 14:04:42 +0800
From:   Ying Xue <ying.xue@...driver.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
CC:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        <tipc-discussion@...ts.sourceforge.net>,
        Jon Maloy <jon.maloy@...csson.com>
Subject: Re: [Patch net v3] tipc: call start and done ops directly in
 __tipc_nl_compat_dumpit()

On 09/05/2018 01:49 PM, Cong Wang wrote:
> On Tue, Sep 4, 2018 at 8:25 PM Ying Xue <ying.xue@...driver.com> wrote:
>>
>> On 09/05/2018 05:54 AM, Cong Wang wrote:
>>> __tipc_nl_compat_dumpit() uses a netlink_callback on stack,
>>> so the only way to align it with other ->dumpit() call path
>>> is calling tipc_dump_start() and tipc_dump_done() directly
>>> inside it. Otherwise ->dumpit() would always get NULL from
>>> cb->args[].
>>>
>>> But tipc_dump_start() uses sock_net(cb->skb->sk) to retrieve
>>> net pointer, the cb->skb here doesn't set skb->sk, the net pointer
>>> is saved in msg->net instead, so introduce a helper function
>>> __tipc_dump_start() to pass in msg->net.
>>>
>>> Ying pointed out cb->args[0...3] are already used by other
>>> callbacks on this call path, so we can't use cb->args[0] any
>>> more, use cb->args[4] instead.
>>
>> It's a common mechanism to save rhashtable iterator pointer in cb->args
>> after tipc_dump_start() and tipc_dump_done() are introduced. Someday
>> probably we will involve new dumpit function. In order to lower the risk
>> that rhashtable iterator pointer saved is overwritten, it's better to
>> use the last slot, ie, cb->args[5].
> 
> I don't understand, currently only cb->args[0..3] are used at most,
> therefore cb->args[4] is pretty safe in current code base, there is
> no reason to be so defensive to use cb->args[5].
> 

Yes, at present cb->args[4] is safe.

> If you really worry about future, you probably want to extend cb->args
> from 6 to whatever larger, rather than just skipping cb->args[4].
> 

When we have to use the fifth slot of cb->args[] in the future, we need
to skip sb->args[4], which is a bit wried. This is the reason why I
suggested we could use the last one.

> I don't see any reason to do so.

As I said, the current version is safe. If you think it's unnecessary to
change, it's okay to me.

Acked-by: Ying Xue <ying.xue@...driver.com>

>